Preheat the oven to 180 degrees C. Grease a large loaf tin or grease and line with parchment.
Cook the chopped mushrooms in a non-stick frying pan till they lose their liquid and all liquid has evaporated. Remove from heat.
Combine the zucchini, breadcrumbs, cheese, oil, onion, egg yolks, garlic, seasoning, marjoram, parsley, salt and pepper. Add the cooked mushrooms. Stir well to combine. If the mixture is too wet, add a little more breadcrumbs.
Beat the egg whites till stiff. Carefully fold into the zucchini mixture.
Spoon the mixture into the prepared loaf tin. Smooth out the surface with the back of a spoon.
Bake in the preheated oven for 1 hour. Remove from oven and let cool in the tin. Run a knife along the edges and turn out to cool completely while still warm. Enjoy once completely cool.